The entry of Chlamydia species into host cells in culture and their subsequent fate will be studied with respect to the following topics: (1) the structural differences between the phagosomal membranes surrounding intact chlamydiae and ordinary phagosomal membranes that are responsible for the inhibition of the fusion of lysosomes with chlamydia-containing phagosomes; (2) the mechanisms whereby non-lymphogranuloma venereum strains of C. trachomatis enter mouse fibroblasts (McCoy cells) in the absence of centrifugation; and (3) the mechanisms by which C. psittaci establishes persistent infections in cultures of mouse fibroblasts (L cells).